The present invention relates to a protecting apparatus for protecting an electronic device provided in a vehicle having a storage battery and a solar battery and driven by power supplied from these sources.
In recent years, vehicles in which part of the electrical equipment thereof, such as ventilating fans, is driven by a solar battery have been proposed. Such a vehicle is disclosed in, for example, Japanese Patent Publication No. 51451/1984.
Since the output of the solar battery varies in accordance with the amount of sunlight, it cannot be supplied as a stable voltage. Hence, the electrical equipment of the type which employs the output voltage of the solar battery as the driving power source is generally driven by a combination of the solar battery and a storage battery mounted on the vehicle. The output voltage of the storage battery mounted on the vehicle is passed through a constant voltage circuit which is operated at a predetermined reference voltage (for example, V.sub.DD =5 volts), and is then applied to a load circuit (a control unit of the electrical equipment) as the constant voltage (see Japanese Patent Laid-Open No. 172016/1989).
At a solar battery power input portion of the load circuit, a voltage monitoring circuit constituted by, for example, a resistance type potential dividing circuit, is provided to monitor the output voltage V.sub.SC (for example, V.sub.SC =0 to 20 volts) of the solar battery. A predetermined constant voltage circuit controls the voltage on the basis of the results of the monitoring so, that the constant voltage can be supplied to the load circuits.
Such a constant voltage circuit or a monitoring circuit generally employs active devices, such as transistors or ICs. These active devices are designed to be driven by power supplied from the storage battery, because they may be operated during the night time when the solar battery cannot be operated. Generally, CMOS devices are used as these active devices because of their low power consumption.
FIG. 1 is a circuit diagram illustrating how an output of a solar battery is input to a CMOS device 100 which constitutes part of the aforementioned constant voltage circuit or monitoring circuit. The CMOS device 100 is driven by the constant voltage V.sub.DD obtained from the output of the storage battery, as stated above. The switching device 100 monitors V'.sub.SC into which the output V.sub.SC of solar battery is divided by resistors R.sub.m, R.sub.n on an input line 102. A protective diode 101 protects the CMOS device 100 when the voltage on the line 102 exceeds V.sub.DD. Generally, the monitoring circuit is formed of the CMOS device 100, protective diode 101 and resistors R.sub.m, R.sub.n.
When the power supplied voltage V.sub.DD to the CMOS device lowers to a predetermined reference voltage or less, V'SC &lt;&lt;V.sub.DD occurs. In such a situation, an overcurrent will flow through the diode 101 and diode 101 will be damaged. Furthermore, an upper FET circuit of FIG. 1 is reverse-biased and the monitoring circuit will not work properly due to the deterioration of the FET.
For the reason described above, a CMOS device has a critical maximum rated voltage, that is a voltage which must not be exceeded so that the device may be assured to properly operate. The critical maximum rated voltage is generally around V.sub.DD +0.3 volt.
In the circuit of FIG. 1, the values of the resistors R.sub.m, R.sub.n are set in advance so that V'.sub.SC might not exceed 5.3 volt, preferably, V'.sub.SC =5 V.
If the output voltage of the storage battery lowers to a predetermined reference voltage or less due to over discharge or removal of the battery, the constant voltage circuit cannot control the voltage, and the critical maximum rated voltage of CMOS device 100 goes to 0.3 volt. In such a situation, where there is an amount of sunlight on the solar battery, V'.sub.SC goes to 20 volt. Therefore, R.sub.m and R.sub.n which are set by a manufacturer of the CMOS 100 cannot prevent V'.sub.SC from exceeding the critical maximum rated voltage=0.3 volt, thereby, resulting in the breakage of the device.